Abstract
A new method of mosaic fate mapping, called focusing, is introduced. Its advantages are that it allows a mapping, on the blastoderm surface, of the site of action of functions defined by either pre-adult lethal or behavioral mutations. Moreover, it does not require that the mosaics used be 50% of one genotype and 50% of the other. Methods for quantitative evaluation of the results of focusing, and a comparison of this method with others, are discussed.—Focusing is applied to the analysis of a new mutant, doomed (symbol: dmd), a distally located X-linked recessive in D. melanogaster, that causes the deaths of males and females around the time of eclosion. The dmd phenotype among eclosing flies is first seen as the loss of thoracic motor coordination and as ether sensitivity. Fate mapping by the method of focusing places the site of action of the dmd+ function in the same region of the map as that of the thoracic neural ganglia primordia, but not that of muscle, suggesting the possibility that the effect of dmd is on a thoracic neural function rather than a muscular one.
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